Devices are known containing rechargeable batter having service times, i.e. the time between charging periods, of one hour to many hours. Most of these devices are equipped with a low battery alarm signal that may be audible or visual or both that indicates to the user that the batteries are nearly depleted and must be recharged promptly. The user may then attach the unit to a charger (which is connected to an AC outlet), or connect an internal charger to an AC outlet, for a recommended minimum time, after which the user may remove the unit from the AC outlet and expect the unit to be charged. Since the infusion pump may be administering life-sustaining fluids to the user, it is preferable to have the battery recharge time be as low as possible.
It is known in the art that rechargeable batteries may be recharged quickly by applying a "fast" charging current, i.e. a current approximating the maximum output current "C" of the battery. Unfortunately, along with the benefit of quicker charging comes the disadvantage of a potentially dangerous or battery-damaging condition or both. Battery life can be significantly shortened and excessive heat is often generated by overcharging the batteries. To avoid this situation, the known devices use a "trickle" charging current that is significantly less than C, perhaps C/10, such that the danger and possible damage are minimized, while tolerating the increased recharging time. "Quick" charging current levels are also known that are higher than a trickle charging current, but are still significantly less than C. The charging time is decreased with the quick charging current as compared to the trickle charge, and the danger and potential battery damage are reduced as compared to recharging the batteries at a current near C, but are not completely eliminated.
Thus, battery charging devices and methods exist that attempt to reduce the recharging time safely by using a plurality of charging currents during one recharging session. Such devices and methods are disclosed in U.S. Pat. Nos. 4,710,694; 4,394,611; and 4,240,022. One known method is to apply a quick charging current followed by a trickle charging current. The quick charging current can be applied until, for example, a time limit has run out, or a certain battery voltage has been reached, after which time the trickle charging current is applied.
One device is known that makes use of a quick charging current for recharging the batteries in an ambulatory medical device. This device, the FRESENIUS Frenta System has a feature described as "Rapid Charge." This feature allows the user to select a quicker charging current whenever desired. The higher charging current is then applied for a predetermined period of time, after which a trickle charge is applied.
Unfortunately, by giving the user control over the quick charging, several undesirable conditions can occur. For example, the user can select the quick charge and allow the batteries to be fully charged. The user removes the unit from its charging base, turns it on for ten minutes (a small fraction of the battery service time and power) and then reconnects the unit to its base and recharges the unit with another quick charge. The charging base will apply a quick charging current, rapidly bringing the batteries to their full charge level, while several hours of quick charge time remain. Although the danger is not excessive, the batteries will be damaged over time, greatly reducing their reliability, and eventually leading to battery failure.
It is thus an object of the invention to provide an improved battery charging device and method that decrease overall charging time, while protecting the batteries from damage.
It is a further object of the invention to provide a battery charging device and method that are automatic and immune to the inaccuracies of user control.
It is a further object of the invention to provide a battery charging device and method that increase the battery service time.